In the case of reducing optical imaging, in particular in the field of projection lithography, the image-side numerical aperture NA is generally limited by the refractive index of the surrounding medium in image space adjacent to the image surface of the projection objective. In immersion lithography the theoretically possible numerical aperture NA is limited by the refractive index of the immersion medium.
The immersion medium can be a liquid or a solid. An immersion liquid is typically disposed between an exit surface of the projection objective and the surface of the substrate to be exposed, which is arranged in the image surface. In contact-free solid immersion a planar exit surface of the projection objective is arranged at a working distance smaller than the operating wavelength to the substrate to be exposed such that evanescent fields emerging from the exit surface can be used for imaging (near-filed lithography). Solid immersion with touching contact between the exit surface of the projection objective and the substrate is also possible.